In hospital and clinical establishments, as well as in research situations, it is frequently necessary to slowly withdraw blood samples from a human patient or an experimental animal over an extended period of time. This is conveniently accomplished by inserting a cannula into a blood vessel, such as a vein or an artery, of the patient or animal and withdrawing the blood as desired from the cannula. In order to prevent blood coagulation in the cannula or in associated tubing for conducting the blood sample from the cannula, it is usual practice to mix an anticoagulant material, such as heparin, with the blood sample as soon as possible after it is withdrawn from the blood vessel. For this purpose a double lumen cannula apparatus has been used for many years. In this double lumen apparatus a smaller diameter tube is inserted within the usual cannula sheath. The passage through this smaller diameter tube forms one lumen, and the annular space between the outer wall surface of the inner tube and the inner wall surface of the cannula sheath forms the other lumen. The inner tube terminates within the cannula sheath a short distance from the tip of the cannula sheath. In use, the anti-coagulant material passes through the annular lumen toward the tip of the cannula sheath where it contacts and mixes with the blood entering the cannula from the blood vessel. The mixture of blood and anti-coagulant material then flows out of the cannula through the inner tubular lumen of the double lumen cannula apparatus.
The prior art double lumen cannula apparatus, had two principal disadvantages. First, the distance between the end of the inner tubular lumen and the tip of the cannula sheath was not controlled with any high degree of precision. Most double lumen cannula apparatus have a support body through which the inner tubular lumen and a conduit for anti-coagulant material are passed. The cannula sheath which normally has a funnel portion at one end and a tubular portion at the other end is mated in a conical press-fit against the body so that the inner tubular lumen is located coaxially within the tubular portion of the cannula sheath. Any variations in the length of the cannula sheath and/or of the inner tubular lumen or of the conical press-fit between the cannula sheath and body can cause a variation in the distance between the end of the inner lumen and the tip of the cannula sheath. If this distance is undesirably low, some of the anit-coagulant material from the annular lumen might undesirably enter the blood vessel of the patient or animal. There would also be inadequate dilution of the blood sample by the anti-coagulant material. This can cause false readings when the diluted blood sample is subsequently analyzed. If the distance is undesirably large, some of the blood start to coagulate in the cannula sheath before it contacts the anti-coagulant material.
This undesirable variation in distance is achieved typically in two ways. Manufacturing tolerances in the length of the cannula sheath and/or of the inner tubular lumen can result in such variation. This can be partially overcome by restricting the manufacturing dimensional variations that will be accepted. When the cannula sheath mates against the support body, any variation in the final position of the mated parts from a desired position can cause the above undesired distance variation. In the prior art apparatus the cannula sheath generally mates in a conical press-fit over an extension of the main support body. As in the case with most conical press-fit joints, there is often considerable variation in the final position. This is caused both by tolerance in the tapers of the sheath and body and also by variations in manual pressure employed to mate the sheath against the body. One prior art apparatus employs a resilient material for the support body and its extension. The resiliency of this structural element can also cause undesirable dimensional variations in the assembled apparatus.
The second disadvantage of the prior art apparatus is in the excessive volume of the introduction chamber for the anti-coagulant material. In a typical double lumen cannula apparatus the space in the funnel portion of the cannula sheath between the support body and the junction between the funnel and tubular portions of the cannula sheath forms an introduction chamber for such material. In normal use the cannula sheath is inserted first into a blood vessel. Blood begins to flow through the tubular portion and into the funnel section of the cannula sheath. The catheter portion consisting of a support body, a first conduit forming the inner tubular lumen and a second concuit to provide the anti-coagulant material is then inserted into the cannula sheath with the cannula sheath mating against the support body. The volume of blood initially in the cannula sheath and especially in the funnel portion occupies the space intended for introduction of the anti-coagulant material and should be minimized so that it will not begin to coagulate before being contacted by the anti-coagulant material. This volume has considerable variation in the prior art apparatus.
There is thus a need for double lumen cannula apparatus having improved control over the dilution of a blood sample with an anti-coagulant material and also having minimal volume for a chamber introducing the anti-coagulant to the cannula sheath.